The present invention relates to an ion source and a mass spectrometer, and to mass spectrometry employing the ion source, as well as to a monitoring system employing the same or to a monitor employing the ion source.
Conventionally, as a method of detecting a minor component in a gas or a liquid with a high sensitivity, it has been known to detect ions generated by ionization in a measuring sample with a high sensitivity by means of a mass spectrometer.
There are various methods for ionizing a sample. One of various sample ionizing methods is an atmospheric pressure chemical ionization method employing a corona discharge. Japanese Patent Application Laid-Open No. 51-8996 (1976) discloses a method in which a sample is introduced in a corona discharge region, which is generated at the tip end of a needle electrode by applying a high voltage thereto, for ionizing the sample. At this time, in addition to the case where the sample is directly ionized by a corona discharge (primary ionization), the sample is also ionized by ion molecule reaction (secondary ionization), resulting in high efficiency ionization of the sample molecule.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. 6-310091 (1994), among ionization methods which use corona discharge, there has been proposed a method of ionizing a sample without directly introducing a sample gas into a corona discharge region. Namely, the method proposes to use primary ions generated in a separately provided corona discharge region and to efficiently perform secondary ionization of the sample molecules not passing through the corona discharge region by ion molecular reaction. This method is particularly effective for an objective sample, such as silane gas, which cannot be directly ionized using a corona discharge without extreme contamination of the ion source by the discharge product.
Furthermore, there is a method disclosed in U.S. Pat. No. 4,023,398 to French et al. In the disclosed method, a curtain of gas is blown between the corona discharge region and an aperture for introducing ions into the vacuum region in order to prevent introduction of a carrier gas into the mass spectrometric portion which is under vacuum conditions for transporting the sample. Thereby, the discharge efficiency can be improved when a vacuum discharge system, such as a cryopump, is used.
Furthermore, Japanese Patent Application Laid-Open No. 3-179252 (1991) discloses a gas flow direction. In this method, a liquid sample flows through a hollow needle electrode, and a fine droplet of the liquid sample ionized at the tip end of the needle is efficiently vaporized by a dry gas flowing in opposition thereto. On the other hand, dispersion of the ionized sample is suppressed to improve the maintenance efficiency. However, Japanese Patent Application Laid-Open No. 3-179252 (1991) does not disclose a problem in the stability of the discharge, nor a solution of such a problem.
In the method disclosed in the above-identified Japanese Patent Application Laid-Open No. 51-8996 (1976) and Japanese Patent Application Laid-Open No. 3-179252 (1991), when the concentration of the measuring sample is low (for example, upon measurement of a minor component in the air or upon measuring a minor component in a liquid), the intensity of ions of the component present in a greater amount in comparison with the ions of the measuring sample, or ions originated from a component present in a large amount (for example, an ion or the like generated by ion molecular reaction), becomes extremely high. Accordingly, when the sensitivity of a detector is adapted to the ions of the measuring sample in a fine amount, ions of the component present in a large amount or ions originated from the component present in a large amount may reach the detector to cause a large current to flow, resulting in damage to the detector, thereby to gradually degrade the amplification factor of the current.
On the other hand, when a molecule corresponding to a component present in a large amount or a molecule corresponding to ions originated from the component present in a large amount can be ionized easier than the molecule of the objective sample, the generation efficiency of the ions of the objective sample is lowered, thereby lowering the sensitivity. Furthermore, in case of an ion trap type mass spectrometer performing mass spectrometric analysis of accumulated ions by scanning a high frequency voltage after accumulating the ions, ions less than or equal to a mass number corresponding to the amplitude of the high frequency voltage to be applied may directly reach the detector after passing through an ion trap mass spectrometric portion. Accordingly, when ions are present in a large amount, the detector may be damaged, thereby to gradually degrade the amplification of the current.
On the other hand, Japanese Patent Application Laid-Open No. 3-179252 (1991) is related to an ion source for analyzing a liquid sample, but has no disclosure for analysis of gas.
Further, the method disclosed in U.S. Pat. No. 4,023,398, while providing for introduction of a carrier gas into a vacuum by using a curtain of gas, the obtained mass spectrum is nothing different from the prior art. Therefore, problems similar to those of the prior art can be expected.
Furthermore, in the prior art set forth above, the components in the gas to be measured may be deposited on the tip end of the needle, so as to make the corona discharge unstable, thereby to cause difficulty in effecting continuous measurement over a long period of time. In case of the method disclosed in Japanese Patent Application Laid-Open No. 6-210091 (1994), nothing has been discussed with respect to continuous operation over a long period, such as one month.
Accordingly, it is an object of the present invention to provide an ion source using a corona discharge for efficiently generating ions of a sample and a device employing the same.
Another object of the present invention is to provide an ion source which can maintain a stable discharge for a long period, and a device employing the same.
At first, as a means for improving ion generation efficiency, in a corona discharge generated at the tip end of a needle electrode by applying high voltage thereto, a direction connecting the needle electrode and a partitioning wall having an opening for passing the generated ions into a mass spectrometric portion, namely a direction along which the ions are drawn from the discharge region, and the direction of flow of a sample gas are different. Thereby, the efficiency of generation of ions of the objective sample can be significantly improved.
By adopting the construction as set forth above, the following effect is obtained. For example, a primary ion molecular reaction in the case where chlorophenols in dry air are monitored using a negative corona discharge becomes as follow:
O2+exe2x88x92xe2x86x92O2xe2x88x92
(negative corona discharge)
O2+N2xe2x86x922NO
(negative corona discharge)
O2xe2x88x92+NOxe2x86x92NO3xe2x88x92
O2xe2x88x92+(Cp)xe2x86x92(CPxe2x88x92H)xe2x88x92+HO2
Here, (CPxe2x88x92H)xe2x88x92 represents a negative ion removed proton from CP. As can be appreciated from the foregoing expression, basically, O2xe2x88x92 generated by negative corona discharge intervenes reaction. By reaction of N2 and O2 under corona discharge, NO3xe2x88x92 is easily generated via the NO as an intermediate product. Thus, ions having a high strength can be monitored. Since NO3xe2x88x92 has a high acidity, it will not react with CP. Accordingly, what is monitored, when the concentration of N2 is quite high in comparison with CP, is mostly NO3xe2x88x92, and little (CPxe2x88x92H)xe2x88x92 is monitored.
Among such reaction processes, when the reaction of O2xe2x88x92+NOxe2x86x92NO3xe2x88x92 can be restricted, the reduction of O2xe2x88x92 can be suppressed. By this, O2xe2x88x92+CPxe2x86x92(CPxe2x88x92H)xe2x88x92+HO2 is progressed. Thus, NO36xe2x88x92 possibly generated in a large amount can be significantly reduced. Accordingly, the amount of the objective (CPxe2x88x92H) which is generated can be increased. For restricting generation of NO3xe2x88x92, it is important not to form overlapping regions of O2xe2x88x92 and NO. One approach for this is to make the direction of ion movement different from the direction of the intermediate product flow by use of an electric field, thereby to make the period during which the intermediate is present in the corona discharge region extremely short. Particularly, when the foregoing two directions are opposite, a significant effect can be achieved. At this time, in the reaction process set forth above, the presence of the intermediate product NO can be ignored. Accordingly, the foregoing reaction becomes substantially:
O2+exe2x88x92xe2x86x92O2xe2x88x92
(negative corona discharge)
O2xe2x88x92+CPxe2x86x92(CPxe2x88x92H)xe2x88x92+HO2
This mode of reaction is quite desirable from the point of view of monitoring (CPxe2x88x92H)xe2x88x92 with a high precision.
The present invention provides a mass spectrometer and relates to mass spectrometry. In a mass spectrometer, having an ion source performing ionization of a sample by generating a corona discharge at the tip end of a needle electrode through application of a high voltage to said needle electrode, the ion source is constructed so that the angle between the direction, in which the ions pass from the needle electrode to the opening for introducing the generated ions into the mass spectrometric portion, namely the direction along which the ions are drawn from the discharge region, and the direction of flow of the sample gas is greater than or equal to 90xc2x0. (In other views, a line extending from the first opening for feeding ions to the mass spectrometric portion to the tip end of the needle electrode, and a line extending in the direction of sample gas flow into the discharge region form an angle which is less than or equal to 90xc2x0.
With the construction set forth above, in addition to improvement of the ion generation efficiency of the measurement objective substance, a stable discharge can be maintained for a long period. One of the reasons for this will be discussed in terms of a negative corona discharge.
The reason why a stable discharge cannot be maintained for a long period when a gas flow is not present in the discharge region is that components in the gas are deposited on the tip end of the needle electrode to make the curvature of the tip greater, causing instability in the discharge. In such a case, fluctuation is caused in the current value of the discharge to make the ionization unstable and the ion strength measured by the mass spectrometer also fluctuates. In order to perform an analysis with high reliability, maintenance to frequently change the needle electrode and sharpening the tip end of the needle electrode is required.
When a negative corona discharge is used, an electron which is discharged from the tip end of the needle electrode with a negative high pressure is caused to collide with a neutral molecule in t he gas to cause ionization. The ionizing region is quite close to the tip end (within 1 mm). The negative ion generated in this way moves toward the counter electrode in response to the electrical field generated between the needle electrode and the counter electrode. The phenomenon whereby the radius of curvature at the tip end of the needle electrode becomes greater, results in fluctuation of the discharge current value for the f ollo wing reasons.
1) The negative ions generated in the ionizing region in the vicinity of the tip end of the needle electrode may weaken the field intensity in the tip end region of the needle electrode.
2) When the field intensity is weakened, the discharge current drops.
3) The negative ions generated in the ionizing region move toward the counter electrode in response the force of the electric field.
4) The field intensity in the tip end region of the needle electrode becomes enhanced again.
By repetition of the foregoing steps 1) to 4), the discharge current flows. When the radius of curvature of the tip end of the needle electrode is small, even if the field intensity is weakened, the discharge current may not be lowered to permit continuous discharge. However, if the radius of curvature of the tip end of the needle electrode is increased, due to the effect of the space charge of the negative ions, the field intensity is lowered, which results in lowering of the discharge current. When the voltage is lowered in order to prevent lowering of the discharge current, the discharge mode may transit from corona discharge to spark discharge, thereby to make discharge mode unstable, which is not suited for analysis.
When the flow of gas is produced in the ionizing region, the negative ions which possibly cause a space charge are dispersed to restrict the lowering of the field intensity at the tip end of the needle electrode.
Accordingly, by generating a gas flow at the tip end region of the needle electrode, stable discharge can be maintained even when the radius of curvature of the tip end of the needle electrode becomes large due to deposition to a certain extent.
When the sample gas flows from the root side of the needle, the region for actively causing ionization in the vicinity of the tip end of the needle electrode becomes disturbed, thereby to cause difficulty in efficiently dispersing the space charge.
On the other hand, by providing a function for monitoring the discharge current value, when the amplitude of the current value becomes greater than or equal to a certain value, an alarm can be given for exchanging the needle, the whereby maintenance timing for replacement of the needle electrode can be determined properly.